run-tests: echo message and exit if no tests found

The test runner currently defaults to running the entire test suite if
no selected tests can be found. This can be unexpected.

For example the ext/mysqlnd/ directory has no tests, if you specify that
directory when testing the entire test suite will be run.

    run-tests.php [options] ext/mysqlnd/

Closes GH-5605.

Author: gerardroche

run-tests.php

@@ -394,6 +394,7 @@ function main()
     $temp_urlbase = null;
     $conf_passed = null;
     $no_clean = false;
+    $selected_tests = false;
     $slow_min_ms = INF;
     $preload = false;
     $file_cache = null;
@@ -634,6 +635,7 @@ function main()
         }
 
         if (!$is_switch) {
+            $selected_tests = true;
             $testfile = realpath($argv[$i]);
 
             if (!$testfile && strpos($argv[$i], '*') !== false && function_exists('glob')) {
@@ -664,6 +666,11 @@ function main()
         }
     }
 
+    if ($selected_tests && count($test_files) === 0) {
+        echo "No tests found.\n";
+        return;
+    }
+
     // Default to PHP_BINARY as executable
     if (!isset($environment['TEST_PHP_EXECUTABLE'])) {
         $php = PHP_BINARY;